The render result is correct but when in the viewer node the render is going off screen
Without having your .blend and image files at hand, I can't tell for sure what is happening. 

However, usually a "Scale" node (found under "Distort"), set to "Render Size" and applied just after the Image input, should guarantee that RenderLayers and external images have matching sizes. (Note: if the image is smaller than the render layers, it will be stretched, losing sharpness in the process) 



I don't know, however, why they don't match already if – as you say – you already changed the resolutions in both the image and Blender. I can just suggest to check again.